1. Field of the Invention
The present invention relates to a light emitting device which uses a light emitting element utilizing electroluminescence, and particularly, a lighting system.
2. Description of the Related Art
A light emitting element is a self-light emitting element and attempted to be used as a lighting system. The lighting system using the light emitting element can provide lighting systems tailored to customers' needs such as high efficiency and excellent color rendering properties.
It is said that a light emitting mechanism of the light emitting element emits light by applying a voltage between a pair of electrodes where a light emitting layer is interposed, so that electrons injected from a cathode and holes injected from an anode are recombined with each other at a light emitting center of the light emitting layer to form molecular excitons and the molecular excitons release energy when returning to a ground state. Singlet excitation and triplet excitation are known as excited states, and light emission can be achieved through either of the excited states.
As for such a light emitting element, there are many problems depending on materials in improving element characteristics, and improvement in element structure, development of materials, and the like are conducted to overcome the problems.
On the other hand, as one element structure, a light emitting element having a structure in which a plurality of light emitting units divided by charge generation layers are stacked between an anode and a cathode opposing to each other has been reported to achieve longer life when emitting light with high luminance (refer to Reference 1: Japanese Patent Laid-Open No. 2003-45676 and Reference 2: Toshio Matsumoto, Takeshi Nakada, Jun Endo, Koichi Mori, Norihumi Kawamura, Akira Tokoi, Junji Kido, IDW '03, 1285-1288). This charge generation layer needs to be formed of a material having a function to inject carriers and having an excellent light transmitting property.
In order to achieve white light emission in the structure disclosed in References 1 and 2, there are structures, for example, in which a blue light emitting unit and an orange light emitting unit are stacked as shown in FIG. 8A, and in which a blue light emitting unit, a green light emitting unit, and a red light emitting unit are stacked as shown in FIG. 8B.
However, a light emitting element provided with a plurality of light emitting units has a problem causing color shift where an emission color of the light emitting element changes as time passes unless light emitting units to be combined with each other have the same length of life. The color shift is a phenomenon in which a proportion of luminance of each light emitting unit is changed and an emission color as the light emitting element is changed. As for a decrease of luminance, constant luminance can be maintained by increasing a voltage to be applied, but color shift cannot be solved only by adjusting a voltage to be applied. For example, when the life of a blue light emitting material is shorter in FIG. 8A, an emission color gradually becomes yellowish white as shown in FIG. 8C after a certain period of time with the light emitting element lighted.